The present invention relates to a circuit arrangement for the limitation of over-voltages in energy storage modules composed of energy storage elements. The circuit includes a series circuit of “n” energy storage elements, wherein the series circuit is connected between a first potential and a second potential and voltage-limiting elements are associated with the “n” energy storage elements.
Energy storage modules serve for the storage of electrical energy. The storage capacity of conventional energy storage elements is usually smaller than the required total amount of energy to be stored. The maximum permissible voltage across an energy storage element is usually less than the voltage required for the application. Accordingly, several energy storage elements with smaller energy storage capacity are combined to form an energy storage module. Energy storage modules are usually designed as capacitors or accumulators. Super-capacitors (SCAP) form a special kind of capacitors. These super-capacitors are also termed thin-layer capacitors, ultracaps, boostcaps or supercaps. Super-capacitors of that kind have increasing significance as electrical energy stores for a number of applications, particularly in automobile construction or in portable electrical appliances, in which very high load quantities have to be stored or released rapidly.
However, an individual super-capacitor can be operated only with a permissible voltage Umax of 2.3 V to 2.5 V, so that for most cases of use several super-capacitors have to be connected together in a series circuit and the energy storage module with a maximum voltage of N·Umax is formed. Super-capacitors are subject to a production tolerance which can result in a capacitance fluctuation of −10% to +30%. Moreover, capacitance losses of approximately 20% can arise due to ageing. The ageing process of super-capacitors is temperature-dependent and thus not equal for all super-capacitors contained in an energy storage module. An unequal voltage distribution between the individual energy storage elements results therefrom in the case of charging of an energy storage module containing super-capacitors. Ultimately, the super-capacitor, which due to the ageing process has the smallest capacitance, in the series circuit is loaded with the greatest voltage. However, there are various procedures for achieving a uniform voltage distribution and avoiding damage of the super-capacitors.
One possibility is selecting the super-capacitors, which are used in an energy storage module, by measuring the capacitance and a corresponding selection of the super-capacitors with small production tolerances. This has the disadvantage that selected super-capacitors of that kind with smaller tolerance are accompanied by higher production costs. Moreover, this approach does not offer any security against ageing effects, since even super-capacitors with small production tolerance are subject to ageing and exposed to losses of capacitance. Thus, the super-capacitors used in an energy storage module can be damaged, since the voltages at the individual capacitors are different.
A further possibility for protection against over-voltages due to losses of capacitance consists in operating an entire energy storage module at a lower overall voltage. However, this is disadvantageous because the entire energy storage module is not utilized efficiently. In order to avoid over-voltages at the super-capacitors it is also possible to connect, in parallel with the super-capacitors, resistances which are, however, accompanied by a high discharge current and thus produce during operation an energy loss which is consciously taken into account. If the resistances are selected to be too large, then the symmetry gain is thereby impaired. A further possibility of counteracting damage of super-capacitors of that kind in an energy storage module consists in using an active over-voltage shunt with Zener diodes. For this purpose the Zener diodes are each connected in parallel with a respective super-capacitor. However, this has the disadvantage that Zener diodes with a breakdown voltage of 2.5 V have, due to their relatively soft diode characteristic curve, equally a large spontaneous discharge below the breakdown voltage and thus are not efficient. Alternatively to use of an active over-voltage shunt by means of Zener diodes it is also possible to use integrated threshold value circuits which guarantee an active over-voltage limitation. However, threshold value circuits of that kind are usually constructed as integrated circuits and consequently relatively expensive.
A current storage installation with batteries and capacitors, in which super-capacitors are used, is shown in publication WO 02/15363 A2. As illustrated above, super-capacitors cannot take over-voltage and thus have to be monitored. In the WO 02/15363 A2 publication several different wiring variants for avoidance of over-voltages are described. There is a description of connecting a Zener diode across several super-capacitors, wherein, however, only the voltage across all super-capacitors lying in parallel with the Zener diode is limited not across a super-capacitor alone. The individual super-capacitor is thus not protected against over-voltages.
If, apart from the Zener diode connected in parallel across several super-capacitors, there is no additional protective wiring then the super-capacitor with the smaller capacitance has a greater voltage after a charging process. If the Zener diode limits the voltage to twice the respective permissible voltage of the super-capacitors the super-capacitor with the smaller capacitance is overloaded.
Ageing usually has the consequence of a decrease in capacitance. Moreover, the ageing processes advance in an accelerated manner with increased voltage, i.e. it is necessary to reckon on a halved service life in the case of a voltage increase by 0.1 V. There is thus a disastrous loop: The smaller capacitance leads to an increased voltage. The increased voltage leads to more rapid ageing and this in turn to an accelerated capacitance decline. In the case of renewed charging this element is then loaded with an even higher voltage and thus ages more rapidly. This means that the asymmetry is then amplified up to the point of total failure of the super-capacitor. This is because the smallest capacitance value in the series circuit determines the overall capacitance.